High schooler Shungo Ninomiya, a capable martial artist, shares a large house with his older sister Ryouko, who works as a mercenary. Ryouko moves the Tsukimura siblings in with him: Mayu, a young succubus whose terror of men is as strong as the pull her own nature has on them, and her incubus brother Mikihiro. Curing Mayu's phobia lands on Shungo, starting with the two of them sharing a room.